I was taught that in general Van Der Waals and usually weaker than hydrogen bonds but at some point can become stronger if the size of the molecules is big enough. I was wondering if this was correct, and if so, how would you find out when it becomes stronger?

It is important to note that hydrogen bonds are just a special type of polar bonds that are stronger because of the large difference in electronegativity between hydrogen and oxygen, nitrogen, and fluorine. Thus, hydrogen bonds result from a permanent dipole whereas van der Waals forces are induced dipole-induced dipole. Hydrogen bonds are much stronger than van der Waal's forces (-20kJ/mol vs. -2kJ/mol).